1. Field of the Invention
The present invention discloses a method of compensating signal imbalance of a wireless communication system, and more particularly, a method of reducing signal imbalance called I/Q imbalance occurring in a wireless communication system by generating a set of filter coefficients utilized by the wireless communication system and by generating compensation signals using the set of filter coefficients to process wireless signals.
2. Description of the Prior Art
Please refer to FIG. 1 and FIG. 2, which illustrate a transmitter 110 and a receiver 120 of a conventional communication system. The transmitter 110 includes digital-to-analog converters 112 and 132, low-pass filters 114 and 134, mixers 116 and 136, a local oscillator 117, an adder 138, a power amplifier 119, and an antenna 118. The receiver 120 includes an antenna 128, a low noise amplifier 149, mixers 122 and 142, a local oscillator 127, low-pass filters 124 and 144, gain controllers 126 and 146, and analog-to-digital converters 129 and 148. The antenna 128 is configured to receive signals transmitted from the antenna 118. The digital-to-analog converters 112 and 132 respectively receive an in-phase portion I of a signal and a quadrature portion Q. Under an ideal condition, a phase difference between the quadrature portion Q and the in-phase portion I is 90 degrees, i.e., the quadrature portion Q and the in-phase portion I form a pair of mutually-orthogonal signals. On top of that, the mutually-orthogonal signals are also balanced in amplitude, i.e., there is no amplitude mismatch in the mutually-orthogonal signals either.
In the transmitter 110 shown in FIG. 1, signals of the local oscillator 117 are mixed into mixers 116 and 136 to form a primary source of signal imbalance between the quadrature portion Q and the in-phase portion I, i.e. the so-called I/Q imbalance, while processing the quadrature portion Q and the in-phase portion I. The I/Q imbalance caused by signals of the local oscillator 117 sabotages the orthogonality and the amplitude balance between the quadrature portion Q and the in-phase portion I and causes interference between the quadrature portion Q and the in-phase portion I. Besides, in the receiver 120, the I/Q imbalance also occurs between the quadrature portion Q and the in-phase portion I because of signals of the local oscillator 127 mixed into the mixers 122 and 124, where the I/Q imbalance is irrelevant to frequency.
There is also a frequency-related I/Q imbalance between the transmitter 110 and the receiver 120, where the frequency-related I/Q imbalance is caused by mismatch between analog elements of the transmitter 110 and the receiver 120, and the analog elements may include low-pass filters, analog-to-digital converters, or digital-to-analog converters.
Please refer to FIG. 3, which schematically illustrates the I/Q imbalance occurring in the transmitter 110 shown in FIG. 1 and the receiver 120 shown in FIG. 2 in a frequency domain. As shown in FIG. 3, if frequencies of the signals outputted from the transmitter 110 are equal to a frequency fRF1=fLo+fm1 and a frequency fRF2=fLo+fm2, where the frequency fLo is the frequency of the local oscillator, besides the signal portions Sm1 and Sm2 respectively having frequencies fRF1 and fRF2, image portions Im1 and Im2 respectively having frequencies (fLo−fm1) and (fLo−fm2) as shown in FIG. 3 are also generated because of the I/Q imbalance occurred between the quadrature portion and the in-phase portion in the transmitter 110 and the receiver 120. Besides, when the receiver 120 receives signals having a frequency fRF1=fLo+fm1 and a frequency fRF2=fLo+fm2 from the transmitter 110 via the antenna 128, since the I/Q imbalance is also introduced by the local oscillator 127, the receiver 120 generates four signals having different frequencies fm1, fm2, −fm1, and −fm2 in order, where signals having the frequencies fm1 and fm2 indicate the signal portion in the receiver, and the signals having the frequencies −fm1 and −fm2 indicate the image portion in the receiver 120. If the image portions Im1 and Im2 respectively having frequencies (fLo−fm1) and (fLo−fm2) can be filtered off, and if the image portions Im1′ and Im2′ respectively having frequencies −fm and −fm2 can also be filtered off, i.e. if the orthogonality and amplitude balance between the quadrature portion and the in-phase portion of signals in the transmitter 110 and the receiver 120 can be kept without interfering each other, noises caused by the I/Q imbalance in the transmitter 110 and the receiver 120 can be neutralized.